CD97 is a G-Protein Coupled Receptor which is mainly expressed on hematopoietic lineage cells and is upregulated as an early response gene in activated T and B cells. The extracellular region of CD97 contains 3-5 EGF-like domains and an RGD motif, suggesting a role for CD97 in cell-cell or cell-matrix interactions, although a specific ligand for CD97 which would fufill such a role has not been identified. We are focusing our efforts on establishing 1)the biological role that CD97 plays in immune function, and 2) defining signaling pathways activated by ligation of CD97. In order to address function, we have produced a CD97 null strain of mouse. Early investigations have shown normal development and fertility for these mice. Ongoing studies address the response of CD97 null mice to various forms of immune challenge. We have found that the expression of CD97 in CHO and 3T3 cells was in itself sufficient to cause JNK phosphorylation and Pyk dephosphorylation. Over-expression of a receptor may amplify a weak basal signal above detectable thresholds. Using deletion mutants, we discovered that such signaling required only the membrane proximal domain of the extracellular region in addition to the first transmembrane domain of CD97, inconsistent with a G protein coupled mechanism. The physiological relevance of this observation remains to be established.